This invention was made in the course of, or under, a contract with the Energy Research and Development Administration. It relates generally to a process for improving the mechanical properties of gas cooled reactor fuels.
Fuels for high temperature gas cooled reactors have generally been composed of spherical particles of fissile uranium or plutonium as an oxide for carbide in the form of an approximately spherical kernel. The kernal may also be formed of a solid solution such as (UPu)C.sub.2 or (UPu)O.sub.2. This kernel sometimes is additionally composed of a fertile material such as thorium-232 or uranium 238, which is in the same chemical form as the fissile material. Alternatively the fertile material can be in a separate particle from the fissile material. The fertile and fissile fuel kernels are also provided with several layers of protective coatings to contain fission products and to protect the fuel kernels. The kernel, along with the coatings, constitutes a fuel particle. Normally, these coatings will comprise a buffer layer of porous carbon within a layer of dense pyrolytic carbon, followed by a layer of silicon carbide, and a final layer of dense pyrolytic carbon. For kernels containing only fertile material it is common to use a two layer coating, the so called Biso design, consisting of a buffer and dense carbon layer. The inner or buffer layer of porous carbon with about 30 to 70 percent porosity absorbs any expansion or swelling of the kernel during irradiation and minimizes damage to the other layers due to fission fragment recoil from the kernel. The adjacent dense carbon layer is applied to isolate the kernel from attack by deleterious gases such as chlorine formed in depositing the silicon carbide layer. The silicon carbide layer gives a dimensional stability to the overall fuel particle and provides containment for metallic fission fragments. The final dense pyrolytic carbon layer protects the silicon carbide plus provides a rough surface to permit fuel rod fabrication. The combination of all of the layers serves the function of operating as a pressure vessel for containment of fission product gases. Fuel particles are normally approximately spherical or spheroidal with a diameter of about 400 to 1200 micrometers. The central fuel kernel is generally about 200 to 1000 micrometers in diameter. The layer of porous carbon is generally about 25 to 100 microns in thickness. The inner and outer layers of dense carbon and the silicon carbide layers are each typically 20 to 50 micrometers thick. In the case of two layer designs the buffer and dense carbon layers are each typically 50 to 120 micrometers thick.
The various layers of coating are applied to the fuel kernel by techniques which are well known in the prior art. Generally these layers are applied while the kernels are suspended within a fluidized bed such as that described in U.S. Pat. No. 3,889,631. The highly porous buffer carbon layer is deposited by the thermal decomposition of acetylene as is disclosed, for example, in U.S. Pat. No. 3,472,677. This decomposition occurs while the particles are suspended within a gaseous medium. The acetylene is mixed with the suspending gas which is generally an inert gas such as argon. The silicon carbide layer is similarly deposited within the same fluidized bed by the thermal decomposition of methyl trichlorosilane. Dense carbon layers are generally applied by the method described in U.S. Pat. No. 3,471,314 from the thermal decomposition of propylene but mixtures of propylene and acetylene, methane, and other hydrocarbons have been used. These various coating operations are generally carried out within the following temperature ranges:
Buffer carbon coating 1000.degree. to 1500.degree. C
Silicon carbide coating 1400.degree. to 1700.degree. C
Dense isotropic carbon coating 1200.degree. to 1500.degree. C.
While all of these prior art processes produce a product which is satisfactory for use in a gas cooled reactor, the processes themselves tend to produce a high percentage of defective particles. Since there is no effective process for culling defective particles, the released fission products complicate maintenance of contaminated reactor components. Cracks appearing in the second coating layer, i.e. the pyrolytic carbon, are the primary cause of defective particles. In the case of the fissile particles the crack lets chlorine generated during silicon carbide coating reach the kernel while for the two layer coating design a crack results in escape of an appreciable fraction of the fission products. In addition to the high percentage of cracked particles the structurally intact particles which emerge from the coating operation have coating layers with high stress levels. Prior art attempts at solving these problems have been only partially successful. Such attempts have included a subsequent annealing step. This subsequent annealing step has somewhat reduced the problem of high stress levels within the coating layers. However, this step does nothing to reduce the percentage of cracked particles.